Phase interpolating is needed in various applications, such as in clock and data recovery (CDR). Phase interpolators are often included in various types of phase locking loops (PLL) which may be used, for example, in the radio front end, or in the radio transmitting stage, of a mobile terminal device.
Key factors determining the performance of a phase interpolator include low noise of the interpolated phase and good linearity of the interpolation operation carried out by the circuit(s) of the phase interpolator.
Phase interpolators, typically implemented as integrated circuits (IC), often comprise basic building blocks, or interpolating circuit units, carrying out the actual interpolating operation. Such circuit units may be combined and connected together to form stages, which in turn may be connected to form multi-stage interpolators.
Noise and linearity performance of a phase interpolator is strongly dependent on the performance of the interpolating circuit unit(s) thereof. One of the most commonly used types of interpolating circuit units is that utilizing constant slope charging of a capacitor, providing good linearity of the interpolating operation. Basically, a constant slope interpolating circuit unit is based on charging a capacitor by current sources.
The performance of a phase interpolator being based on constant slope interpolating circuit unit(s), as well as that of phase interpolators of other types, may be greatly dependent on match between the components, such as current sources, used in the interpolating circuit unit(s).